The present invention relates to a method for audio video signals' multiplexed transmission system using a interphone cable made of a pair of parallel PVC-coated wires.
Interphone whose power consumption is relatively small usually uses dry cells as the driving power source. When a TV door phone system is used in place of the existing interphone, however, the door phone camera in the outdoor unit cannot be driven by the dry cell provided for supplying power to the interphone, because the camera requires larger power. Moreover, when replacing the interphone with the TV door phone, it is necessary to newly install another power cable for driving the door phone camera and a co-axial cable for video signal transmission, resulting in extra installation cost. For the purpose of saving power-supply cables there is already known a method that with a co-axial cable for video signal transmission DC power may be transmitted, superposed on video signals by mean of series regulators for duplexing of DC power with a video signal at the sending terminal and for de-duplexing at the receiving terminal. In general, spectra of both audio and video signals include such low-frequency components as 100 Hz at least, so that power choke coils are necessary for DC power duplexing but these are too heavy, bulky and power-dissipative to compcatly fabricate the indoor or outdoor unit.